(1) Field of Invention
Molding of settable plastic material.
(2) Description of Prior Art
The method commonly used for the molding of articles of plastic materials, such as vulcanizable rubber and thermo-setting and thermoplastic materials, employs compression molds consisting of top and bottom plates or sections having mating cavities in each of which a core pin is supported with proper clearance. The plates are supported with the parting surfaces horizontal and the uncured material to be molded is introduced into each cavity by placing a first semi-solid sheet thereof in the bottom mold cavity under the core pin and a second like sheet over the core pin. The mold plates are disposed in a conventional press with their parting surfaces or planes horizontal, closed under the pressure applied by the press, and then heated or otherwise treated to cure the material therein.
In this prior method of compression molding material in excess of that required for the article must be introduced into each cavity in order to develop proper molding pressure on the material when the mold is closed, the excess being squeezed out as flash before the mold plates reach fully closed position. Even with an exceptionally good mold and careful charging there is always some flash. With horizontally disposed cavities, spillage of part of the charge is a problem. The flash material which is squeezed out of the cavities into the space between the mold plates, or into flash accumulating passages or pockets disposed at the parting surface of the plates, is relatively thin and begins to set quickly. As a result this flash material developes low flow and compressive characteristics which resist full closure of the mold plates and require very high total press pressure to drive the plates close to fully closed position. If too much material is introduced a higher pressure results and more is produced around the entire article. If not enough material is introduced, the mold cavity may be insufficiently filled and the material may be molded under insufficient pressure with the result that defective articles are produced. If the material is unsymmetrically distributed in an open mold, even with the proper charge, much of the charge can escape from the mold cavity as flash leaving insufficient material to be distributed and fill the cavity upon closure of the mold.
Unless the number of cavities is limited, this resistance exceeds the capacity of the press to close the molds fully and produce precisely shaped and dimensioned articles with a small amount of flash. In any event, the articles tend to have a degree of porosity due to the limitation of pressure on the material in the cavities resulting from the excess being permitted to escape as flash. Further, the dead weight of the molds limits the number which can be handled in the conventional vertically stacked condition by an individual workman. Careful material preparation is necessary for product consistency and considerable time and expense in production are required for material preparation and subsequent removal of flash. The amount of flash is considerable and constitutes a very substantial waste of material.
As one example, in the prior methods commonly utilized in molding tubular golf club handle grips, molds of about two feet square with approximately fourteen cavities per mold are used, and require a curing cycle of approximately eight to ten minutes. Normally, not more than two molds stacked vertically flatwise are used during each press cycle because their combined weight exceeds the power of a workman to manipulate them.
Sometimes the molds are of the transfer type in which the material is charged into a transfer pot and partially plasticized and transferred to the cavities by way of sprue holes.
Another method currently used is the conventional injection molding. Precisely shaped articles can be produced by this method, but flashless articles cannot be produced thereby because the extreme pressures required for injection molding separate the mold plates or sections slightly with the result that flash material is forced into the spaces therebetween. The high pressures are necessitated by the fact that the runners connected with the sprue, or sprues, are usually of small cross section and, in turn, connect with very small gates leading into the mold cavities. For example, the gates are often about 1/32 of an inch in diameter. Runners and gates of this nature are desirable because of the frictional and shearing effects they have on the material being injected, which effects raise the temperature of the material and maintain it at the higher temperature and in better plasticized condition in the mold. Because of these effects, the heat control must be precisely balanced in relation to the flow characteristics of the runners and gates if consistent filling of all of a large number of cavities concurrently is to be realized. If the material introduced into the sprue is too warm, the additional frictionally generated heat added to the initial heat often over-cures the material. If the material being injected is not sufficiently warm, the material cannot pass through the runners and gates properly to assure filling of the cavities. Further, such molds must necessarily be exposed to very high holding pressures by the press, due to the reactive forces imposed by the high pressure of the material in the runners, cavities, and spaces between mold plates at the parting surface into which the material escaped. As a result air and gases cannot escape readily and the entrapped air results in the formation of defective and blistered articles.
Since the plasticizable material must be preheated to a high temperature and kept at that temperature to assure a proper flow through the small runners and gates, a long cooling cycle is required. The number of molds that can be filled with one injection is limited thereby and by the capacity of the plasticizing cylinder.